In general, OLEDs (Organic Light Emitting Diodes) have the advantages of low power consumption, fast response time, wide viewing angle, and so on. The OLEDs have a simple basic structure and can be easily manufactured, and thus are used for ultra-thin, ultra-light weight display units of 1 mm or less thick. Further, the OLEDs are fabricated on flexible substrates such as plastic substrates instead of glass substrates, and thus are also used in thinner, lighter, and unbreakable flexible display devices.
The OLED is formed of a multi-layered organic thin film including an anode electrode, a hole transport layer, an organic light-emitting layer, an electron transport layer, an electron injection layer, and a cathode electrode.
In addition, an organic solar cell is formed of a multi-layered organic thin film including an anode electrode, a hole transport layer, a photoactive layer, an electron transport layer, an electron injection layer, and a cathode electrode.
It has been widely known that the OLED and the organic solar cell having such a configuration, are formed by using a vacuum deposition method to form an organic thin film such as a hole transport layer, an organic light-emitting layer (or a photoactive layer), an electron injection layer and an electron transport layer.
When manufacturing an organic thin film by a sputtering or vacuum deposition process, a vacuum chamber is required. Accordingly, a manufacturing process thereof is complicated, a production cost therefor is very expensive, it is difficult for mass production thereof, it is not suitable for manufacturing a flexible OLED, and its application range is not wide.
The electron injection layer is formed by depositing an organic ultra-thin film of about 1 nm made of LiF, CsF, NaF, or Cs2CO3 or a layer of about 20 nm made of Ca, Li, Ba, Cs, or Mg. However, such an electron injection layer is highly susceptible to oxygen and moisture in the outside air even during further deposition of a negative electrode, to accordingly have a serious problem of shortening the lifetime of a device, and making it difficult to handle these materials during a manufacturing process.
In addition, since an organic ultra-thin film electron injection layer uses an organic thin film of 0.5˜2 nm thick in an electron injection layer/cathode structure such as a LiF/Al structure which is the most widely used, the surface condition of a coating film of an underlying layer is very important in which the organic ultra-thin film electron injection layer is deposited on the underlying layer.
Therefore, since it is not easy to have a sufficient coating performance by using a solution process such as a roll-to-roll printing process, an ink jet printing process, a screen printing process, a spray coating process, or a dip coating process, it is very difficult to apply the organic ultra-thin film electron injection layer to the solution process.
To solve this problem, development of an organic thin film forming apparatus using a spray method has been currently actively conducted in order to form an organic thin film by spraying a solution on a substrate with the spray method.
As disclosed in Korean Patent Publication 10-1172187 on Aug. 27, 2012, a method of producing an organic thin film transistor using a conventional spray method includes the steps of: preparing a substrate; manufacturing a solution to be deposited on the substrate; placing a shadow mask on the substrate, in which a pattern is formed on the shadow mask; coating the manufactured solution on the shadow mask through a spray device; and evaporating a remaining solvent by performing heat treatment after terminating the coating step.
Such a method of manufacturing an organic thin film transistor using a conventional spray method may form a uniform organic thin film because of using the spray method, but requires a separate heat treatment apparatus and a heat treatment process since the solvent contained in the solution is evaporated through a separate high temperature heat treatment after having applied the solution on the substrate, to thereby cause the manufacturing process to become complicated and the manufacturing cost to increase.
Also, conventionally, because a high temperature heat treatment is performed in a state where a solution is applied on a substrate to control a volatilization rate of a solvent, the substrate or thin film (solute) may cause a thermal damage by high temperature. In addition, when a thermal expansion coefficient difference between the substrate and the thin film is large, the applied thin film may peel off from the substrate. In addition, when the temperature of the substrate increases, and thus the solvent is rapidly evaporated, closed pores may be generated in the thin film, and thus the solvent in the pores is not sufficiently evaporated, or when the solvent is rapidly evaporated, the surface roughness of the organic thin film becomes high.
Furthermore, since the conventional spray method may cause a thin film which is already deposited on the substrate to be dissolved by the solvent contained in the injected liquid droplets, there may be problems of lowering uniformity of the thickness of the thin film and increasing the surface roughness of the thin film.